This invention relates to addition polymerization. More particularly, the invention relates to the control of outgassing of materials formed addition polymerization. The invention has particular utility when used in a mounting process for outgassing-sensitive optics. The invention may be used for specifying and defining the vacuum quality of polymer material used in the fabrication and manufacture of interferometers.
Successful mounting of optics has been accomplished by various mechanical methods. Of these, bonded optic mountings can usually be effected more quickly and less-expensively than traditional mechanical clamping methods. The structural adhesives most frequently used to hold optics to mounts and to bond mechanical parts together are two-part epoxies, urethanes, and room-temperature-vulcanizing (RTV) elastomers. Adhesives emit volatile ingredients during cure or if exposed to vacuum or elevated temperatures. The emitted materials may then condense as contaminating films on nearby surfaces, such as optics, opto-mechanical assemblies, or items under process.
For all high vacuum applications, cross-linked polymers are most preferred because of extremely high molecular weights generated through the chemical reaction. Such polymers do not outgas except for trace amounts of residual components or unreacted low molecular species left in the polymer network. The two most frequently used polymers are epoxies and silicones. In some instances silicones are preferred over epoxies because of their ability to perform under extreme range of temperatures; however their adhesive properties with various surfaces are not as good as the epoxies. The epoxies and silicones typically used in current fabrication processes meet the users"" current needs but may not necessarily meet the outgassing requirements set by the customer future generation optic assemblies (e.g., interferometers used in high vacuum environments).
For epoxy resins, proper formulation and mixing of the resin with the amine curing agent in stoichiometric proportions is critically important to meet the outgassing requirements under vacuum conditions ranging below 10xe2x88x926 torr. Commonly used amine curing agents under ambient conditions have some vapor pressure and therefore a slight excess or improper mixing would present serious problems and would not meet the vacuum compatibility and outgassing specifications for the optic assembly. To address this potential issue, some epoxy formulations use amine adducts as curing agents. This approach is ideally suited for high vacuum applications. One distinct advantage of amine adduct curing agents is that it is a product of poly-functional amines and mono-functional epoxy intermediates and have much lower volatility due to higher molecular weight. As expected, this curing agent produces a superior vacuum compatibility epoxy end product with low-outgassing performance.
One area of concern is the outgassing of the materials used for optic assemblies in a vacuum environment. Typically the outgassing species include the following:
Trapped and dissolved gases, including nitrogen, oxygen and water vapor;
Possible solvents used the cleaning process and adsorbed gases;
Low molecular weight hydrocarbons and amines that were present as impurities in the epoxy resin and the curing agent; and
residual and unreacted epoxy and amine components.
When building optic assemblies, optic adhesives are typically measured and mixed as needed. Assemblers are required to dispense an exact amount of the various constituents of two-part epoxies, urethanes, or RTVs. These constituents are then mixed, often in small quantities, then applied to the optic for mounting. The problem is getting exact adhesive mixing ratios with this method. And while this method is often adequate for many standard optic applications, newer applications have very tight requirements for outgassing and contamination.
According to the present invention, outgassing of addition polymerized materials is limited by precisely controlling the admixture of the addition polymerized materials. The present invention achieves a significant reduction in the amount of material that outgasses from an optic bond. This, in turn, reduces the amounts of materials that may condense as contaminating films on critical surfaces. Specifically, by choosing different bonding agents, the amounts of total mass loss (TML) and volatile condensable materials (VCMs) can be controlled. This is accomplished by using pre-mixed and frozen (PMF) adhesives in stoichiometric proportions for optic bonding.
According to one embodiment of the present invention, PMF adhesives are used in stoichiometric proportions for optic bonding where outgassing and contamination requirements are severe. A representative application is the construction of interferometers, which are employed in high vacuum, extreme ultraviolet (EUV) lithography stages. The invention also proposes a pass criteria method for accepting or rejecting high vacuum (HV) adhesives and other materials. There are several advantages to doing this:
Stoichiometric mixtures of adhesives are consistently mixed and employed;
Assemblers do not need to mix adhesives as part of their optic assembly process;
Adhesive volatiles are outgassed in a clean room, away from critical equipment;
Optic assemblies are conditioned in a temperature-controlled, ultra-high vacuum (UHV) environment;
A pass criteria is available independent of mass, area, or specific vacuum chamber;
Candidate adhesives can be tested and measured using a standard, quantitative method; and
Low-outgassing adhesives can be appropriately chosen for use in demanding HV and EUV environments minimizing risk to customer equipment.
According to the present invention, standard or custom, high-performance addition polymerizing adhesives are used in relatively large batches and the components of the adhesives are pre-mixed in the large batches. The batches are then packaged in desired quantities for storage until use. By using larger quantities, constituent measurement errors are minimized. By way of example, a 1 gram error has much less effect on a 1000 gram batch than a 10 gram batch. Reducing measurement errors has been shown to reduce the amounts of unwanted outgassing constituents, which typically include hydrocarbons, plasticizers, and silicones. The desired quantities are then dispensed into convenient packages (e.g., 5 cubic centimeter syringes). To keep the pre-mixed adhesives from curing, the containers are frozen at temperatures ranging from xe2x88x9240 to xe2x88x9280xc2x0 C. They are preferably kept at this temperature until ready for use.